The invention relates generally to optical measurements. In particular, the invention relates to methods and instrumentation for characterizing pulse broadening in optical fibers.
The signal capacity of an optical fiber communication system is limited by several fundamental effects, many of which involve the broadening of an optical pulse as it traverses a fiber from the transmitting to the receiving end. Most optical signals have a finite spectral width, that is, range of wavelengths constituting the optical pulse, and the speed of propagation on a fiber varies with wavelength. The wavelength dependence of the speed of light on a fiber is referred to as chromatic dispersion although intra-modal dispersion is a more accurate term. Intra-modal dispersion includes material dispersion and waveguide dispersion. Material dispersion is caused by the variation with wavelength of the optical constants of the materials constituting the optical fiber, particularly its core. Waveguide dispersion arises from the variation with wavelength of the optical fields associated with the waveguide geometry. For example, the diameter of the field and the numerical aperture of any waveguide mode varies with the mode""s wavelength. Material and waveguide dispersion are present in both single-mode and multi-mode fiber.
Although single-mode optical fiber is predominantly used in long-haul communication networks, multi-mode optical fiber offers several advantages when installed in short-haul systems. The larger core is easier to align, which is important for installing local area networks (LANs), which require a large number of fiber connections to be made. Also, multi-mode fiber is compatible with the less expensive detectors such as vertical cavity surface emitting lasers (VCSELs) that emit in the 850 nm band of silica fiber.
A multi-mode fiber is distinguished by its ability to convey (propagate) at least two optical modes while a single-mode fiber can propagate only a single fundamental mode at any wavelength. Degenerate or nearly degenerate polarization modes, the latter of which produce polarization mode dispersion (PMD), will not be considered here. Multi-mode fiber is subject additionally to inter-modal dispersion in which the different modes even of the same optical wavelength propagate at different speeds. Inter-modal dispersion is usually referred to as differential mode delay (DMD).
The manufacture of high capacity optical fiber requires frequent monitoring in the production plant of the bandwidth and differential mode delay to assure that manufacturing and material variations do not unduly limit the fiber bandwidth. The bandwidth (BW) is used to determine the maximum rate of communication, usually stated in gigabits per second (Gbs) or gigahertz (GHz) for a given length of fiber that a given combination of fiber, fiber length, and laser can transfer to a receiver without significant error. For the fiber manufacturer, the relevant BW value is that associated with the impulse response of the fiber itself and is more generally expressed for any length of fiber in units of Gbsxc3x97km. The performance of multi-mode fiber communication systems is often limited by DMD. From DMD measurements of the multi-mode fiber, the bandwidth performance of a particular laser-fiber combination can be predicted. Also, the results from DMD measurements can used to tune the process for creating the fiber preforms such that the drawn fiber will have a maximal bandwidth for use with lasers of various types and characteristics.
However, the presence of significant intra-modal dispersion on a multi-mode fiber, for example, a contribution of more than 10% to the pulse broadening, will interfere with the accurate measurement of DMD or BW determined by inter-modal dispersion. Heretofore, fiber measurement method for time-resolved pulses have not simultaneously measured and corrected the effects of intra-modal dispersion.
A further complication arises from the limitations of pulsed laser sources. The more economical laser diodes have a small but significant spectral width on the order of a few nanometers. The large spectral width of the source is multiplied by the intra-modal dispersion of the fiber in determining the chromatically widened pulse width at the receiving end of the fiber. Furthermore, a pulsed diode laser may be subject to significant chirp in which the spectral content of the pulse changes over the short pulse. The effects of chirp are similar to the pulse broadening of intra-modal dispersion, but chirp occurs prior to the signal being launched on the fiber.
Conventionally, DMD and BW measurements have been performed by impressing a short optical pulse on one end of a long test fiber and measuring the intensity I(t) of the pulse at the other end as a function of time. For bandwidth measurements, a Fourier transform I(xcfx89) of the time profile is compared to the Fourier transform Iref(xcfx89) of the input pulse, usually measured over a short reference fiber. The deconvolution in frequency space of the test pulse and the reference pulse I(xcfx89) and Iref(xcfx89) allows the BW to be computed, as per FOTP-51: Pulse Distortion Measurement of Multimode Glass Optical Fiber Information Transmission Capacity, Electronic Industries Association, EIA/TIA-455-51A, May 1991. The impulse response of the test fiber is the deconvolution, that is, the inverse Fourier transform.
In the past, in the absence of separate consideration of the intra-modal dispersion, four competing requirements have been imposed on the optical pulse that is launched from a laser into the fiber to be tested in order to minimize intra-modal dispersion and other chromatic effects:
(1) a narrow spectral width is required to minimize the error arising from chromatic dispersion arising in the test fiber, which would influence the BW and DMD measurements;
(2) a very short pulse length, typically 50 to 100 ps;
(3) high pulse power required to measure long lengths of fiber and provide a large dynamic range in the measurements; and
(4) low or no chirp in the pulsed laser source.
These requirements attempt to provide an optical source pulse that approaches a delta function in time.
If a laser diode is used for the light source, optimizing any one of the four criteria will, in general, degrade the laser diode""s performance in the other three criteria. The problem becomes acute with 10 Gbs fibers for which pulsed edge-emitting laser diodes no longer can satisfy all three requirements. In particular, measurement errors arising from intra-modal dispersion become significant with ordinarily available diode lasers. Furthermore, the emission spectra of diode lasers tend to be somewhat wide, particularly in the pulsed mode of operation. For multi-mode fibers being developed for 10 Gbs operation, chromatic dispersion arising from the spectral width xcex94xcex for laser diodes of 3 to 6 nm at FWHM accounts for a major fraction of the total pulse spreading in the fiber. As the fiber BW increases, the large intra-modal dispersion leads to a increasingly greater fraction of the error in measuring BW and DMD in the fiber.
The distinct contributions of intra-modal dispersion and differential mode delay are emphasized by differences in operational conditions between factory and field. In the common usage of multi-mode fiber for local area networks, typical maximum lengths are in the range of 300 to 600 m representing a run within a building or between buildings on a campus. Over such lengths, loss is not a significant problem so that low-power laser sources can be used. The characteristics of a vertical-cavity surface-emitting laser (VCSEL) are matched to these conditions. A VCSEL produces a low-power optical pulse with a very small spectral width of about 1 nm or less. Its beam width of between about 10 to 25 xcexcm is easily coupled into a 50 xcexcm multi-mode core. The very small spectral width of the VCSEL source means that intra-modal dispersion is not a significant problem, particularly over the relatively short transmission lengths. In contrast, the factory producing such multi-mode fiber typically winds the fiber in spools of length between 1 and 10 km for shipment and sale. It is greatly desired that factory testing be performed on fiber having such lengths instead of cutting a shorter length or interrupting production. However, multi-mode fiber having a length of greater than 1 km does suffer significant optical absorption and power loss, which increase exponentially with length. Accordingly, a more intense laser source such as a diode laser is needed for such factory testing. However, as mentioned previously, laser diodes have significant spectral widths, typically in the range of 3 to 6 nm for 850 nm laser diodes. As a result, chromatic dispersion significantly contributes to temporal broadening in factory testing even though in the field it tends to not be that important.
Other laser sources are available providing narrow spectral widths and narrow pulses and allow accurate BW and DMD measurements on long fiber lengths, for example, a mode-locked Ti:sapphire laser. Such a laser not only is expensive, but more importantly is a type of laser that may require special eye protections which are difficult to implement in a production environment. However, production line measurements are greatly desired for the high bandwidth multi-mode fiber in order to closely monitor the fiber to be offered for sale.
The invention includes a method of measuring the pulse broadening and other temporal and chromatic characteristics of an optical pulse. The pulse is separated both in time and optical wavelength, and the time- and wavelength-dependent components are recorded.
In one aspect of the invention, the data is then analyzed to chromatically correct it, particularly to reduce or remove the wavelength dependence.
One method of chromatically correcting the data includes calculating the temporal cross-correlation functions between data of different wavelength windows. The temporal offset producing the largest cross-correlation is used to correct the data of that wavelength by time shifting the data by that amount. Further, the multiple temporal offsets producing the largest cross-correlations can be fit to a line, the slope of which measures the intra-modal dispersion.
The corrected pulses may be integrated over wavelength to produces a time-resolved pulse with an improved signal-to-noise ratio.
The corrected data may be used to characterize the optical transmission medium, for example, to establish the differential mode delay of a multi-mode fiber. The invention is particularly useful for monitoring the production of multi-mode fiber.
In another aspect of the invention, the data is analyzed to separate the temporal and chromatic portions of the fiber-broadening of the optical pulse and other types of chromatic and temporal characterization of optical pulses. For example, the inter-modal broadening on a multi-mode fiber may be separated from the intra-modal broadening, and the fiber characteristics may be separated from the characteristics of the pulsed laser source.
A further aspect of the invention includes measuring and compensating for chirp in a laser used to characterize a fiber.
The invention also includes the apparatus for acquiring and analyzing the optical data. The apparatus may include a combination of a spectrograph, a streak camera, and a computer programmed to analyze the acquired data.